That worked better than I hoped for!g_axelsson said:
First I added a few drops of nitric and a drop of hydrochloric acid, in approximately 10:1 proportions. The gold disappeared fast and I got a few drops of yellow liquid. No trace of any damage to the aluminum.
I actually left the AR and gold chloride mixture on the aluminum for over an hour and it still didn't cement back.
First trial, a few drops of nitric with little HCl, a few minutes after adding it.
Stannous test of the wash water from above. The result is in the beaker to the right.
The next day a new test, a few ml of HNO3 and a couple of drops of HCl, this time I used a dropper to the gold covered aluminum bar and let the acid drop back into the beaker. It took longer time as the gold were covered by only a thin film, but eventually the gold were gone and only aluminum left.
Third trial, mixed a new batch of a few ml HNO3 and a bit more HCl. Approximately in 5:1 proportions. Now the acid attacked the aluminum a bit but it dissolved the gold faster. Only on sharp edges are there any gold left, it was hard to get the acid to stay long enough to dissolve the gold.
I saw no cementing of gold in any of the tests. There could have been some at the end. I had a black spot that didn't disappeared until suddenly when it went all yellow and then it dissolved and left clean aluminum. It appears that the nitric acid passivated the surface so no cementing could happen.
Almost done, only a bit left. Dropper and gold chloride in the background.
For once, a plan worked perfectly! 8)
Göran
